MEXICO CITY (AFP) -- The brutal killing of a third person in apparent retaliation for reporting crime on social networks has raised fears of further censorship of Mexico's drug violence.

The beheading of journalist Maria Elizabeth Macias on Saturday came two weeks after the half-naked bodies of a man and a woman were found hanging from a bridge also in the northeastern border city of Nuevo Laredo, along with messages threatening people who report drug violence on the Internet.

The use of social networks to spread information in parts of Mexico is often a matter of survival -- without a political agenda as in China or the Arab revolutions -- because newspapers no longer dare to report on drug violence.

"In the context of violence against the press present in Mexico ... social networks ... break the silence imposed on journalists," the Mexican branch of the Article 19 rights watchdog said in a statement Tuesday.

"That's why it's urgent to guarantee the security of those who use those tools."

Around 10 journalists have been killed this year in Mexico, according to different media watchdogs, alongside eruptions of violence amid a military crackdown on drug gangs that started in 2006. Officials and media reports blame more than 40,000 deaths on drug violence since 2006.

Press freedom groups condemned the killing of Macias, whose decapitated body and head were found near a message citing posts she wrote on a local anti-crime website, "Nuevo Laredo en Vivo."

Tamaulipas state authorities said a criminal group had claimed reponsibility for the crime while news reports said the message was signed 'ZZZZ,' using the letter associated with the Zetas drug gang.

The two bodies found on September 13 were also accompanied by a message signed 'Z.'

The Zetas -- who started as ex-elite army officers working as hitmen for the Gulf cartel in the 1990s -- are blamed for many attacks in Tamaulipas and beyond since they broke off from the Gulf gang in early 2010.

Macias, 39, worked for daily newspaper Primera Hora but her Internet posts were on public forums where locals seek information about gang fights in the area.

Mike O'Connor, Mexico representative for the New York-based Committee to Protect Journalists (CPJ) said it was unclear if her killing was directly related to her Internet activities.

But he added: "there hasn't been trustworthy journalism in Tamaulipas for a long time. Reporters say that if you tell the truth, they simply kill you."

As social networks have become a vital crutch to citizens in violent areas, authorities have expressed concern about their potential to spread fear.

The government of the eastern state of Veracruz last month arrested a man and a woman and accused them of terrorism for spreading rumors of attacks, which turned out to be false, on Twitter and Facebook.

The pair were finally freed last week without being charged, but state authorities also passed a new law -- which rights groups have criticized -- to punish the crime of spreading false messages which "disturb public order."

Even as the law was announced, local social networks ran warnings about the daylight dumpings of 49 bodies on roads around the port city of Veracruz last week.

Conversations on Nuevo Laredo en Vivo's Twitter account on Tuesday highlighted how local press had ignored the killing of Macias.

"She did nothing more than tell the truth," lamented @brujitaaaaaa.